A Multiple Input Multiple Output (MIMO) technology plays an important role in the improvement of peak value and system spectrum efficiency, so conventional Long Term Evolution (LTE) or LTE-Advanced (LTE-A) radio access technologies are built on the basis of an MIMO+Orthogonal Frequency Division Multiplexing (OFDM) technology. In addition, a performance gain of the MIMO technology is derived from a degree of spatial freedom capable of being achieved by a multi-antennae system, so during the standardization, an important evolution for the MIMO technology lies in the extension of dimension.
For a base station antenna system with a conventional Passive Antenna System (PAS) structure, a plurality of antenna ports (each port corresponds to an independent radio frequency (RF)-Intermediate frequency (IF)-baseband channel) is arranged horizontally, and a plurality of antennae elements in a vertical dimension corresponding to each port is connected to each other via a RF cable. Hence, it is merely able for the conventional MIMO technology to optimize horizontal-dimension spatial characteristics of each signal by adjusting relative amplitudes/phases of different ports in a horizontal dimension, but in the vertical dimension, merely a uniform sector beamforming solution can be used. In addition, after an Active Antenna System (AAS) technology is introduced into a mobile communication system, the base station antenna system may acquire a larger degree of freedom in the vertical dimension, so it is able to optimize a signal from a User Equipment (UE) in a three-dimensional (3D) space.
Based on the above, in industry, the MIMO technology is moving in a three-dimensional and massive direction. Currently, the 3rd Generation Partnership Project (3GPP) is studying 3D channel modeling, and in future, it is expected to study and standardize a Full Dimension MIMO (FD-MIMO) technology using more than eight ports (e.g., 16, 32 or 64 ports). In academia, the MIMO technology on the basis of a massive antenna array (including a hundred of, or hundreds of, or more antenna elements) is now being studied and tested proactively. The research and the preliminary channel test result show that, a massive MIMO technology can improve the system spectrum efficiency technically and support more users to access. Hence, the massive MIMO technology has been considered by various research organizations as one of the most potential physical layer technologies for a next-generation mobile communication system.
In addition, in an LTE system, a Cell-specific Reference Signal (CRS)-based transmission and CSI measurement mechanism is adopted by Physical Downlink Shared Channel (PDSCH) Transmission Modes (TMs) 1-7, Physical Downlink Control Channel (PDCCH) for transmitting Layer 1/2 (L1/2) control information and Physical Broadcast Channel (PBCH) for transmitting broadcast information. During the development of the LTE/LTE-A system, along with the separation of the measurement and transmission of the reference signals, the CRS is being gradually replaced with CSI-Reference Signal (CSI-RS) and UE-specific Reference signal (URS) in newly-introduced PDSCH TMs 8-10. However, for the sake of compatibility and the transmission of control and broadcast information, influences on the information transmission and the CSI measurement caused by the introduction of an AAS-based two-dimensional (2D) array need to be taken into consideration.
In a PAS array, each CRS port corresponds to fewer antenna ports. For example, in the case that there are eight bipolar antenna ports and four CRS ports, each CRS port may correspond to two antenna ports. In this case, it is able to conveniently design sector coverage beams adapted to a large angle range, and to ensure the CRS and the CRS-based transmission power efficiency. After the AAS array is used, the number of the available baseband-controllable antenna elements may increase dramatically, so it is able to form an explicitly-oriented narrow beam with for the transmission of services. For the URS-based transmission, the transmission quality depends on the number of the controllable ports and the processing gains from the 3D-MIMO technology. However, for the CRS ports, in order to make full use of their functions so as to ensure a coverage range, the number of the antenna elements corresponding to each CRS port may increase dramatically. Further, in order to meet the requirements of the sector coverage within a wider angle range, merely a non-constant modulus (CM) weight vector can be used. At this time, the power efficiency will decrease, and finally the CRS-based transmission performance and measurement accuracy will be adversely affected.
In a word, for the conventional CRS-based transmission and CSI measurement mechanism, there is a contradiction between the coverage range and the power efficiency in the AAS array. No effective scheme has been currently proposed so as to achieve the 3D-MIMO processing gains based on the AAS technology.